1. Field of the Invention
The present invention relates to working vehicles comprising an HST (hydrostatic transmission), such as tractors.
2. Description of the Related Art
In such types of working vehicles, the engine's power is generally transmitted through a power transmission system comprising a flywheel, a main clutch, transmission gears, an HST, differential gears, a drive shaft, and so on.
Among such working vehicles, frame type vehicles and frameless type vehicles are known. Frameless type vehicles utilize, as a vehicle body frame, a casing in which constituent elements of the power transmission system are accommodated. Frame type vehicles comprise a transmission case in which constituent elements of the power transmission system are accommodated, the transmission case being secured to the main frame of the vehicle body.
For example, such a flame type vehicle is disclosed in Japanese Examined Utility Model Publication No. 1990-39862. Frameless type vehicles are disclosed in Japanese Unexamined Patent Publication Nos. 2002-283860, 2001-105913, and 2002-274203.
In frameless type vehicles, the casing for accommodating the driving force transmission system is generally divided into a flywheel casing for housing a flywheel and a main clutch, an HST casing for housing transmission gears and an HST, and a differential gear casing for housing differential gears. These casings are connected by bolts or the like.
In frame type vehicles, the oil hydraulic pump shaft and oil hydraulic motor shaft of the HST are each supported at one end thereof by a cover casing of the HST, and at the other end thereof by an oil hydraulic circuit block.
In frameless type vehicles, the oil hydraulic pump shaft and oil hydraulic motor shaft of the HST are each supported at one end thereof by an oil hydraulic circuit block secured to the flywheel casing, and supported at the other end thereof by bearings integrally formed with an HST casing (see, for example, Patent Documents 2 and 3), or supported at both ends thereof by bearings integrally formed with an HST casing. The oil hydraulic circuit block has oil passages formed therein.
Furthermore, in such types of working vehicles, in order to provide a sufficient space for the driver's legs, some drive shafts are often positioned low in the drive transmission system by using transmission gears. With such a configuration, transmission gears are generally disposed at the input or output side of the HST. A casing for housing the transmission gears and HST is generally formed into an enclosed oil chamber so that the transmission gears and HST are hermetically sealed therein and working oil can be held in the sealed space.
However, in frameless type vehicles of the prior art in which the oil hydraulic motor shaft and oil hydraulic pump shaft of the HST are supported by bearings integrally formed with a casing that serves both as a vehicle body frame and a casing, the HST is secured while the shafts are supported by the bearings. Therefore, it is troublesome to install the HST. Furthermore, if the shafts of a precision machinery component such as an HST deviate from the axis of the bearing, it often does not operate well in bench tests after the HST installation. In such a case, the assembled parts must be taken apart and reassembled. It is also troublesome to install the HST in frame type vehicles. In both types of vehicles, noise problems may occur during the operation of the HST.
Furthermore, in the case that an HST and transmission gears for reducing the height of the drive shafts are housed in a hermetically sealed oil chamber, entry of dust from the outside can be prevented, but when working oil held in the sealed oil chamber is contaminated with metal powder and sludge due to the wearing of the transmission gears, operational problems may arise because the HST is a precision instrument.